Interesting discussion here. Now allow me to contribute some thoughts.
Note: What you will read below reflects my current understanding of Si and Si type. It can be limited, inaccurate, or incorrect. I’m not a psychologist, not even an amateur one. At this point, I have just begun to read Jung and it takes time to gain comprehension. Also, despite being a non-native English speaker, I will try my best to convey my ideas as clearly and accurately as possible. I welcome any discussions, different views, and/or constructive criticism.
About memory and impression
According to Marriam-Webster,
(I only list the relevant definitions here.)
Memory can be defined as:
1a “the power or process of reproducing or recalling what has been learned and retained especially through associative mechanisms”.
1b “the store of things learned and retained from an organism's activity or experience as evidenced by modification of structure or behavior or by recall and recognition.”
3a. “a particular act of recall or recollection”. 3b. “an image or impression of one that is remembered”. 3c. “the time within which past events can be or are remembered”.
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Here, the keywords include: to recall, to retain, to store, events, images, and impressions.
From these definitions, it seems that memory includes images and impressions. It is likely related to the past events one has witnessed or experienced, thus having a personal significance and subjective flavor.
Impression can be defined as,
1a. “The effect produced by impressing, such as an especially marked and often favorable influence or effect on feeling, sense, or mind.”
2: “an often indistinct or imprecise notion or remembrance.” 3a: a telling image impressed on the senses or the mind.
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Keywords: influence, notion, remembrance, image.
Memory and impression may be used interchangeably sometimes but they do not seem to be exactly the same. Both tend to be related to the sensory and may be detailed, vivid, (in)accurate, biased, or wrong.
Sources:
https://www.merriam-webster.com/dictionary/impression
the power or process of reproducing or recalling what has been learned and retained especially through associative mechanisms… See the full definition
www.merriam-webster.com
Both were accessed on 5/24/2022.
Discussion:
It seems that memory is not specifically related to a particular cognitive function, nor is it an ability accessible only to a particular type. Any type can have strong and vivid memories. However, because personality types indicate preferences, different types may assign different degrees of importance to memory. Some rely on memory and value it, whereas others pay less attention to it. We are not preprogrammed AI with mechanically defined capabilities but rather, we use different cognitive functions to perform various activities. Memory can best be described as an outcome of multiple functions working together, thus it is not only Si that is related to memory.
Next, I’d like to share some thoughts on 1) the understanding of Si; 2) SiNe and NeSi, focusing mainly on the differences, from a layperson’s viewpoint.
To clarify, here the Si type refers specifically to Si-dom, those with Si as their leading function.
The mind of the Si type can be partially compared to a copier. A copier makes copies by producing replicas based on the original document. A decent copier may generate quality replicas whereas a shoddy or malfunctioning one might produce less accurate and less useful copies. While the copier may try its best to produce copies as loyal to the original as possible, the copies and the original cannot be exactly the same.
This means that whether the Si type wants it or not, subjectivity more or less exists, sometimes even beyond their control. How (un)reliable is a copy? It is hardly predictable.
The Si type stores up the copies and relies on them to make decisions, the more copies they possess, the easier it will be for them to make decisions. The Si type’s mind is thus full of replicas of reality, which are all related to the past. This explains why the Si type heavily relies on past experiences and tends to be looking backward most of the time. Any new situation may cause confusion in the mind, as they struggle to find the best copy to match the new situation. If no such copies exist, the Si type tends to turn to others for help. This helps explain why they rely on specific instructions from others and prefer to use templates, follow examples, and perform tasks according to established systems and tried-and-true methods. Such a preference suggests that the Si type may lean more toward J than P.
Also, because of their reliance on past experiences and specific instructions, the Si type tends to be more attentive to the external world and wants to see and experience as much as possible so that they can generate more copies. With limited contact with the world, the Si type is unlikely to thrive. Thus, the Si type may be more in touch with reality – the physical and tangible world than any N-type.
This does not mean that the Si type is less capable of abstract thinking or theoretical discussions. While they might display more suspicion toward anything novel and original, they may be solid theorists as they can effectively synthesize the existing concepts and theories, and use them to explain and analyze something. The Si type may have more faith in authority (the ‘big names’) and mainstream theories. Every individual’s ability, of course, varies. I’m just suggesting that this might be the Si type’s favored way of approaching the abstract.
About SiNe: Suppose Si is a lake and Ne is the fish in the lake. The fish may feel free to roam around everywhere and explore everything, its range of activity, however, is confined within the lake. Si data defines how far Ne can go for an Si type. With Si as the dominant function, Ne tends to be constantly suppressed (like a leashed dog, for example). Thus, the Si type relies heavily on sensory data and past experiences and tends to be less willing to explore the possibilities in the external world. Excessive suppression may cause Ne to go rogue, leading to a deranged mind.
About NeSi: Again, Si is a lake but this time, Ne is a bird living on a tree next to the lake. While the bird treats the lake like home, it can fly away to see more of the world. The Ne type is attentive to the possibilities existing in the external world and not only do they want to know but also have a strong interest in actually trying them. Their Si no longer has the restraining power (it is also much weaker). The Ne type seems to thrive more on new experiences. With Si, while they have impressions from the past, they tend to leave them behind and look forward to "what else is possible". They tend to treat anything from the past with indifference or aversion, because for them, what is new is assigned more significance. Thus, the Ne type can be seen as forward-looking.
Based on Jung’s explanation, it seems that the Si type’s Ne and the Ne type’s Si, are unconscious.